The red abalone, Haliotis rufescens (Swanson 1822), is an important species for commercial aquaculture in Mexico, with annual production levels around 68 t. Not surprisingly, there is great interest in increasing production and its cultivation success. In order to have a better understanding of the genes involved in the sexual differentiation, this study analyzed the early differential expression of eight sex-related genes (VCP 2.2, VERL, VTGI, DMRT1, FP, LYS, SARIP, TEKT A1) by RT-qPCR in abalones from 5 to 45mm of shell length. Sex identification was evaluated using VERL and LYS genes expression levels. These genes differentiated females and males from 16mm of shell length and above. All eight sex-specific genes showed expression in all organisms. However, their level was higher in the sex group to which they correspond. In contrast, differential expression levels occurred in individuals with a shell length of 26-35mm and 36-45mm. These results suggest that sex gene expression is not entirely sex specific, and sexual differentiation in red abalone occurs between 16 and 25mm of shell length. More studies are needed to determine the function of these genes in the sex that they are not associated with.